It is generally known that the temperature in an upper portion of a bore forming body (a bore wall) that defines cylinder bores in a cylinder block becomes higher than the temperature in a lower portion of the bore forming wall. Such non-uniform heat distribution in the bore wall may increase fuel consumption or deteriorate emission. To ensure uniform heat distribution in the bore forming wall, Japanese Laid-Open Patent Publication No. 2002-13440 describes a technique in which a spacer formed of, for example, resin is arranged in a bottom portion of a water jacket (a groove-like cooling passage) of a cylinder block. The technique thus adjusts the flow resistance of the coolant flowing in the water jacket and uniformly cools a bore wall.
However, since the spacer is fitted in the bottom portion of the water jacket, coolant is allowed to flow only in a path located upward from the spacer after the flow resistance of the coolant has been adjusted. It is thus difficult to perform highly accurate temperature adjustment in the cylinder block, or, particularly, in a cylinder bore forming body.
Also, since the spacer is formed of resin with relatively high rigidity, great load must be applied to the spacer when the spacer is inserted into the water jacket and arranged in the bottom portion of the water jacket. This makes it difficult to manufacture a cooling mechanism of an internal combustion engine.